1. Field of the Invention
The present invention relates to an image forming apparatus using electrophotography.
2. Description of the Related Art
For color image forming apparatuses, techniques for reducing hollow defects of characters and thin lines caused at a primary transfer unit have been developed. In a technique disclosed in Japanese Patent Application Laid-Open No. H10-10876, an entrance/exit angle of a belt of a primary transfer unit is differed at an upstream side and a downstream side of rotation. When the transfer unit is a corotron, the transfer unit is arranged not to contact a photosensitive element, and the belt is arranged to make contact with the photosensitive element by applying tension. Because toner transfer is carried out by the transfer unit in a non-contact condition, a press-contact force of the belt is low, and deterioration of image quality caused by, for example, hollow defect of thin lines, is reduced. In addition, since the tension angle is differed at the upstream side and the downstream side, toner scattering before transfer nipping is reduced.
In a technique disclosed in Japanese Patent Application Laid-Open No. H11-38796, a transfer roller is arranged at a downstream side to a photosensitive element, and an auxiliary roller is arranged at a downstream side to the transfer roller, paper that has arrived by being conveyed on the belt can be satisfactorily separated without winding around the photosensitive element. Therefore, a sufficient transfer efficiency can be obtained.
In a technique disclosed in Japanese Patent Application Laid-Open No. 2004-145187, a primary transfer roller is arranged being shifted from a position directly under a photosensitive element, and a voltage is supplied from an identical power supply, unevenness in speed of a belt is reduced. Thus, deterioration in image quality is prevented.
However, if the corotron is used, an amount of ozone emissions is large, which is problematic from a point of view of environmental protection. Moreover, since there are tension rollers in the front and in the rear of a transfer point, a size and cost of the apparatus increase.
In the technique disclosed in Japanese Patent Application Laid-Open No. H11-38796, a toner is directly transferred to a paper from the photosensitive element so that separation thereof is efficiently carried out. However, it is greatly apart from a construction and a purpose of the present invention. Even if a transfer roller is arranged an at a downstream side to the photosensitive element to improve transfer performance, it is disadvantageous since there is a tension roller at a further downstream side to the transfer roller, a size and cost of the apparatus increases.
In the technique disclosed in Japanese Patent Application Laid-Open No. 2004-145187, by providing voltage for a primary transfer from an identical power supply, improvement in belt conveying performance is intended. However, there is no description of an effect on transfer performance itself. Generally, the more downstream side a transfer is carried out, the higher transfer voltage is applied so that a stable toner transfer is achieved. Since the voltage is provided only by the identical power supply, it is considered that transfer performance and transfer efficiency at the downstream side are lowered.